Soft mode driven local ferroelectric transition in lead-based relaxors


Analysis of IR and THz spectra using Bruggeman effective medium approach revealed that the mesoscopic structure of Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Ta2/3)O3  consists of dynamic randomly oriented uniaxially anisotropic polar nanoregions with harder transverse optical polar modes in the direction along the local dipoles.The lowest-frequency phonon of the E symmetry polarized perpendicular to the local dipole moments undergoes softening towards T* ≈ 400 K, which brings evidence about a local structural phase transition. This softening is also responsible for previously observed high temperature dependence of permittivity, which follows the Curie-Weiss law.

Soft modes in PMN, PMT and PMN-PT

(a) Temperature dependence of the overdamped E component of the soft mode in PMN, PMT and PMN-PT characterized by frequency of the peak in dielectric loss.
(b) Temperature dependence of the reciprocal static permittivity 1/ε0 (perpendicular to local polarization in nanodomains) obtained from the fits of IR and THz spectra.

Contact person: Stanislav Kamba